1. Field of the Invention
The present invention relates to an active material for a battery and a method of preparing the same, and more specifically to an active material for a battery with excellent electrochemical characteristics and thermal stability, and a method of preparing the same.
2. Description of the Related Art
Due to recent trends toward more compact and lighter portable electronic equipment, there has been a growing need to develop a high performance and large capacity battery to power this portable electronic equipment. In particular, there has been extensive research to provide such batteries with good safety characteristics and having a low cost.
Generally, batteries are classified as primary batteries, which are used only once before being discarded, and secondary batteries, which are rechargeable for multiple uses. The primary batteries include manganese batteries, alkaline batteries, mercury batteries, silver oxide batteries and so on. The secondary batteries include lead-acid storage batteries, Ni—MH (nickel metal hydride) batteries, nickel-cadmium batteries, lithium metal batteries, lithium ion batteries, lithium polymer batteries, lithium-sulfur batteries and so on.
Lithium ion secondary batteries use materials that reversibly intercalate or deintercalate lithium ions during charge and discharge reactions for both positive and negative active materials, and contain an organic electrolyte or polymer electrolyte between a positive electrode and a negative electrode having the positive and negative active materials, respectively. These batteries generate electrical energy due to changes in chemical potential during the intercalation/deintercalation of the lithium ions at the positive and negative electrodes.
Factors that affect a battery's performance characteristics, such as capacity, cycle life, power capability, safety, and reliability, include electrochemical properties and thermal stability of the active materials that participate in the electrochemical reactions at the positive and negative electrodes. Therefore, there are continuing research efforts to find improvements in the electrochemical properties and thermal stability of the active materials at the positive and negative electrodes.
Of the active materials which have been considered for the active material of the negative electrodes, lithium metal gives both a high cell capacity and a high voltage because the lithium metal has a high electrical capacity per unit mass and a relatively high electronegativity. However, since it is difficult to assure the safety of the battery using lithium metal, a carbonaceous material that is able to intercalate and deintercalate lithium ions is used extensively for the active material of the negative electrodes in lithium secondary batteries. With the use of the carbonaceous material, the battery performance, especially, cycle life and safety, has improved tremendously from that of the lithium metal battery. In order to further improve the negative electrode performance, it has been suggested to add an additive, such as boron, to the carbonaceous material, especially by coating with the additive. For example, a boron-coated graphite (BOC) improves the performance characteristics of the carbonaceous materials.
Lithium metal compounds of a complex formula are often used as a positive active material of the lithium secondary battery. Typical examples include LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCOxO2(0<x<1), LiMnO2 and a mixture of these compounds. Manganese-based positive active materials such as LiMn2O4 or LiMnO2 are relatively easy to synthesize, less costly than the other materials, and environmentally friendly. However, these manganese-based materials have a disadvantage in having a relatively low capacity. On the other hand, LiCoO2 has many technical advantages over the other materials such as relatively good cycle life and relatively high specific energy. This compound is presently the most popular material for positive electrodes of commercially available Li-ion batteries. However, it is relatively expensive. While it is desirable to further improve its stability on charge-discharge cycling at a high rate, it is one of the most stable compounds of the presently available positive active materials. LiNiO2 has the highest discharge capacity of all positive active materials mentioned above, but it is difficult to synthesize and is the least stable among the compounds mentioned above.
Among these compounds, LiCoO2 is the most well accepted in the battery industry since its overall performance characteristics, especially, cycle life, are superior to the others. Accordingly, most of the commercially available rechargeable lithium batteries adopt LiCoO2 as the positive active material, although its cost is relatively high. There is a great deal of research effort in the industry to develop a further improved active material in overall performance as well as to reduce the cost, if possible.
One of the previous efforts includes substituting a part of the expensive Co from LiCoO2 with other less expensive metals. For instance, SONY CORPORATION prepared LixCo1-yMyO2 by doping about 1 to 5 percent by weight of Al2O3 into LiCoO2. A&TB (ASAHI & TOSHIBA BATTERY CO.) prepared an Sn-doped Co-based active material by substituting a part of the Co from LiCoO2 with the Sn.
Another approach is to coat a lithiated compound with a coating material. In U.S. Pat. No. 5,292,601, LixMO2 (where M is at least one element selected from Co, Ni, and Mn; and x is 0.5 to 1) is suggested as an improved alternative material over LiCoO2. U.S. Pat. No. 5,705,291 suggests a method in which a composition comprising borate, aluminate, silicate, or mixtures thereof is coated onto the surface of a lithiated intercalation compound.
Japanese Patent Laid-Open No. Hei 9-55210 discloses coating a lithium nickel-based oxide with an alkoxide of Co, Al and Mn, and performing a heat-treatment to prepare a coated positive active material. Japanese Patent Laid-Open No. Hei 11-16566 discloses coating a lithium-metal oxide with another metal and/or an oxide thereof. The another metal includes Ti, Sn, Bi, Cu, Si, Ga, W, Zr, B, and Mo. Japanese Patent Laid-Open No. Hei 11-185758 discloses coating the surface of a lithium manganese oxide with a metal oxide by using a co-precipitation process followed by heat-treating the same to prepare a positive active material.
In the above description, positive active materials of lithium secondary batteries and related examples of developments were explained. Recently, with demands for more compact and light weight portable electronic equipment, various types of batteries including a Li-ion battery have similar demands for an improved active material that can assure good battery performance, safety, and reliability. A great deal of the research and development efforts have been devoted to improvements on performance and thermal stability of the positive active materials to ensure improved cell performance, safety, and reliability of batteries under various use conditions, including many abuse conditions.